1. Field of the Invention
The present invention relates to a bias circuit used for an amplifier and so on.
2. Description of the Related Art
A bias circuit in an amplifier application, for example, is connected to an AC circuit including an amplifying element, which is an active element, and supplies a DC voltage to the amplifying element without affecting transmission of a signal at a frequency to be amplified.
FIG. 16 shows an example of a conventional bias circuit for an amplifier used in a device such as a radio device. The bias circuit 100p shown in FIG. 16 supplies a DC voltage to a field-effect transistor 180, which is an example of the active element that performs amplification. The bias circuit 100p includes a transmission line 181 one end of which is connected to the gate terminal 185 of a field-effect transistor 180 to which a DC voltage is applied, a capacitor 182 one end of which is connected to the other end of the transmission line 181 and the other end is grounded, a choke coil 183, which is a DC circuit, one end of which is connected to a connection point 186 between the transmission line 181 and the capacitor 182, and a DC power supply 184 which is connected to the other end of the choke coil 183 and generates a constant DC voltage with respect to a ground potential. The line length of the transmission line 181 is one-quarter of the wavelength of an operating frequency.
In the following description, the field-effect transistor is referred to as the FFT and the wavelength of a frequency is denoted by λ.
The capacitance of the capacitor 182 is chosen such that the impedance of the capacitor 182 becomes negligibly small at the frequency of an AC signal with a wavelength λ. As a result, the transmission line 181 corresponds to an end short-circuited transmission line for the AC signal with the wavelength λ. Because the transmission line 181 is ¼-wavelength long as stated above, the impedance of the bias circuit 100p viewed from the gate terminal 185 of the FET 180 in the bias circuit 100p shown in FIG. 16 can be considered to be infinite at the AC signal frequency with wavelength λ. The DC power supply 184 is connected in terms of direct current to the gate terminal 185 of the FET 180 through the choke coil 183 and the transmission line 181. These elements constitute a DC network which applies a DC voltage to the gate terminal 185 of the FET 180. As a result, the FET 180 operates on a DC voltage from the DC power supply 184 as a bias voltage. Such a bias circuit is disclosed in Japanese Patent Laid-Open Application No. 11-150431, for example, which is laid open in 1999. (Patent literature 1)
As the variety of services provided through radio communications increases, dual- or multi-band radio devices capable of handling the information in multiple frequency bands are demanded in these years. In order to enable a radio device to be used under multiple frequency bands, a radio circuit such as an amplifier must be designed to operate at each of multiple frequency bands. One of the demands associated with designing radio circuits such as amplifiers for multiple frequency bands is to enable a bias circuit to operate at each of multiple frequency bands.
For example, Japanese Patent Laid-Open Application No. 2003-101440 (referred to as Patent Literature 2, hereinafter) discloses a technology of using multiple conventional bias circuits to enable an amplifier to operate at each of multiple frequency bands.
Another technology is disclosed in Japanese Patent Laid-Open Application No. 2001-267864 (referred to as Patent Literature 3, hereinafter). The technology employs a configuration where the bias voltage of an active element adapts to working frequency bands, and thereby enables an amplifier to operate at each of multiple frequency bands by changing matching condition on the basis of significantly changing the input and output impedances of the amplifier.